Where a pharmaceutical or cosmetic composition is formulated for external topical application, it will generally also contain, in addition to the “active” component, components which will ensure the composition has appropriate properties for storage, application and skin-surface retention. In this regard, many such compositions currently contain mammalian gelatin, e.g. as a gelling agent.
Gelatin is a derivative of collagen, which is the most abundant protein in animals. Collagen is the major constituent of connective tissue, where it is present as water-insoluble collagen fibers. The general amino acid sequence is Gly-X-Y, where X often is proline and Y often is hydroxyproline. The content of the imino acids proline and hydroxyproline will differ from species to species, and collagen from homoiothermic animals (such as mammals) has a higher content of these imino acids compared to collagen from poikilothermic animals (such as fish).
Gelatin is most commonly produced from collagen from the bone and skin of cattle or pig by acid or alkali extraction procedures. These give, respectively, gelatin type A and type B which have different isoelectric points. The collagen molecule is a right-handed triple helix made up from three alpha-chains. The triple helices, the ordered conformation of collagen, are stabilised by proline and hydroxyproline units in the alpha-chains. The term collagen relates to the unmodified molecules found in the triple helices. Gelatin is the extracted collagen from which the terminal ends of the collagen molecules are lost. The gelling of gelatin is believed to be a result of regeneration of areas of triple helical structure (ordered conformation) interconnected by disordered amino acid segments (random coil). These gels are thermoreversible and both the gelling and melting temperatures are influenced by the content of proline and hydroxyproline. Gelatins from mammals, containing approximately 24% of these imino acids, have gelling and melting temperatures around 20-25° C. and 35-40° C., respectively. Cold water fish species contain only 16-18%, and fish gelatin typically gels below 8° C. and melts at 12-14° C. Hence, gels based on unmodified fish gelatin are only mechanically stable at temperatures corresponding to those found in refrigerators (0-5° C.).
Gelatins have been used for decades in pharmacology, foods and cosmetics as well as in photography, in glues and in composite materials. An increased search for alternatives to mammalian gelatin has been observed over the last decades, especially due to the outbreak of Bovine Spongiform Encephalopathy (BSE), a fatal neurological disorder of adult cattle which also may infect humans in the form of new variant Creutzfeldt—Jacob disease (nvCJD). Additionally, the use of mammalian gelatin is limited by religious concerns; neither Muslims (Halal/Haram), Jews (Kosher) nor Hindus accept gelatin from mammalian sources.